Present day oscilloscopes commonly include an internal sweep trigger generating circuit which is driven by an input signal to produce a train of trigger pulses used to synchronize the sweep signal of the oscilloscope with the input signal. To enable synchronization of the sweep signal with complex input waveforms the internal trigger generating circuits include external manual controls for precisely selecting the portion of the input waveform which is sensed to initiate a trigger pulse. Inasmuch as the sweep signal is synchronized with the trigger pulses, these trigger controls can be precisely adjusted while viewing the display screen of the oscilloscope, and when they are thus adjusted, the sweep trigger pulses occur at the frequency of the input signal.